John P. Collister

Hypotensive Response to Losartan in Normal Rats Role of Ang II and the Area Postrema

We have reported that the angiotensin II (Ang II) AT1 receptor antagonist losartan markedly lowers arterial pressure in sodium-replete, normotensive rats. We hypothesized that this action of losartan was mediated by its blocking the effects of endogenous Ang II. To test this hypothesis, rats were instrumented with arterial and venous catheters for measurement of arterial pressure and infusion of losartan, respectively. After 3 days of control measurements, losartan was infused for 10 days (10 mg/kg/d) in rats on a normal daily sodium intake (NNa; approximately 2 mmol/d, n=6) and rats on a high daily sodium intake (HNa; approximately 15 mmol/d, n=7) to suppress endogenous Ang II. Although basal plasma renin activity was markedly suppressed in HNa rats (0.9 +/- 0.4 ng Ang I/ mL/h) compared with NNa rats (4.0 +/- 0.3 ng Ang I/mL/h), control arterial pressure was not different between NNa (113 +/- 4 mm Hg) and HNa (113 +/- 2 mm Hg) rats. Losartan decreased arterial pressure from control levels in NNa rats on the first day of infusion (-12 +/- 2 mm Hg) but had no effect on arterial pressure in HNa rats (+4 +/- 4 mm Hg). Furthermore, by day 10 of losartan infusion, arterial pressure had decreased further from control levels in NNa rats (-32 +/- 2 mm Hg) but remained unchanged compared with control in HNa rats (+5 +/- 6 mm Hg). A second study was conducted to test the hypothesis that the area postrema, a circumventricular organ proposed to mediate the long-term neurogenic pressor activity of Ang II is a site of action for losartan. After 3 control days, losartan was administered for 10 days to area postrema-lesioned rats (APx; n=11) or sham-lesioned rats (n=10) consuming an NNa diet. Control arterial pressure was similar in sham (95 +/- 3 mm Hg) and APx (96 +/- 2 mm Hg) rats. Basal plasma renin activity was not different between groups (sham, 4.1 +/- 1.5 versus APx, 5.3 +/- 1.6 mm Hg Ang I/mL/h). On day 1 of losartan treatment, arterial pressure decreased to a significantly lower level in sham (80 +/- 2 mm Hg) compared with APx (90 +/- 3 mm Hg) rats. This trend continued through day 4 of losartan infusion, in which arterial pressure in sham rats (72.2 +/- 2 mm Hg) was significantly lower than in APx rats (83 +/- 4 mm Hg). However, during the remainder of the losartan infusion, there were no significant differences between groups with the exception of day 8 (sham, 72 +/- 2 mm Hg; APx, 84 +/- 2 mm Hg). Taken together, these results support the hypothesis that the hypotensive actions of losartan in sodium-replete, normotensive rats are due to blockade of the physiological effects of endogenous Ang II. Furthermore, an intact area postrema is essential for full expression of the hypotensive actions of losartan in normal rats.

The role of Ang (1-7) in mediating the chronic hypotensive effects of losartan in normal rats

Hypothesis The following studies were designed to test the hypothesis that Ang (1-7) contributes to the chronic hypotensive effects of the angiotensin II AT1-receptor antagonist, losartan, in normal rats. Introduction We have previously shown a chronic, hypotensive response to the AT1-receptor antagonist, losartan, in normotensive rats. The mechanism of this response is not completely understood. Previous studies by others have demonstrated a role for Ang (1-7) in the beneficial antihypertensive effects of angiotensin-converting enzyme (ACE) inhibition. This is thought to be due to vasodilatory effects of increased levels of Ang (1-7) during ACE inhibition. Since it has now been shown that Ang (1-7) levels are also increased during AT1 antagonism, we designed experiments to test the hypothesis above. Materials and methods Sprague-Dawley rats were instrumented with venous catheters and radiotelemetric pressure transducers and commenced on a normal (0.4%) NaCl diet. Arterial pressure responses were measured in rats treated with losartan (10 mg/kg/day) (LOS rats, n=8) and compared with those treated with losartan and the Ang (1-7) antagonist, A779 (24 µg/kg/hour) (A779/LOS rats, n=11) for 10 days. Results By day 7 of treatment, mean arterial pressure had dropped by 27±1 mmHg in LOS rats, in contrast with a decrease of only 21±2 mmHg in A779/LOS rats. This attenuated response in rats treated with A779 became more prominent and continued through day 10 of losartan treatment. Conclusion These results support the hypothesis that the chronic hypotensive effects of losartan in normal rats are mediated in part through the actions of Ang (1-7).

The role of the subfornical organ in angiotensin II-salt hypertension in the rat

Hypertension caused by chronic infusion of angiotensin II (Ang II) in experimental animals is dependent, in part, on increased activity of the sympathetic nervous system. This chronic sympathoexcitatory response is amplified by a high‐salt diet, suggesting an interaction of circulating Ang II and dietary salt on sympathetic regulatory pathways in the brain. The present study tested the hypothesis that the subfornical organ (SFO), a forebrain circumventricular organ known to be activated by circulating Ang II, is crucial to the pathogenesis of hypertension induced by chronic Ang II administration in rats on a high‐salt diet (Ang II–salt model). Rats were randomly selected to undergo either subfornical organ lesion (SFOx) or sham surgery (Sham) and then placed on a high‐salt (2% NaCl) diet. One week later, rats were instrumented for radiotelemetric measurement of mean arterial pressure (MAP) and heart rate (HR) and placed in metabolic cages to measure sodium and water balance. Baseline MAP was slightly (but not statistically) lower in SFOx compared with Sham rats during the 5 day control period. During the subsequent 10 days of Ang II administration, MAP was statistically lower in SFOx rats. However, when MAP responses to Ang II were analysed by comparing the change from the 5 day baseline period, only on the fifth day of Ang II was MAP significantly different between groups. There were no differences between groups for water or sodium balance throughout the protocol. We conclude that, although the SFO is required for the complete expression of Ang II–salt hypertension in the rat, other brain sites are also involved.

Role of the Subfornical Organ in the Chronic Hypotensive Response to Losartan in Normal Rats

Abstract—Angiotensin II is known to act at a unique set of brain regions known as the circumventricular organs. These structures lack the normal blood–brain barrier and are therefore thought to participate in the central nervous system processing of neuroendocrine signals. We have reported that chronic treatment with the angiotensin type 1 (AT1) receptor antagonist, losartan, decreases arterial pressure in normotensive rats. Furthermore, this hypotension is attenuated in area postrema–lesioned rats, suggesting a role of endogenous angiotensin II at this circumventricular organ. Another circumventricular organ, the subfornical organ (SFO), has also been shown to mediate actions of angiotensin II. The present study tested the hypothesis that the SFO is a central site of action of endogenous angiotensin II at AT1 receptors. Adult male Sprague-Dawley rats were anesthetized and placed in a stereotaxic apparatus, and the SFO was sham or electrolytically lesioned. One week later, rats were instrumented with venous catheters and radiotelemetry pressure transducers for continuous infusion and monitoring of mean arterial pressure, respectively. After 3 days of control, losartan was administered intravenously (10 mg · kg−1 · d−1) for 10 days in both SFO-lesioned and sham rats. By day 4 of losartan administration, mean arterial pressure had decreased to 75±2 mm Hg in sham rats (n=9) but had only fallen to 83±2 mm Hg in lesioned rats (n=10). This attenuated hypotensive response in SFO-lesioned rats continued through day 10 of losartan treatment. These results support the hypothesis that the SFO mediates part of the hypotensive effects of chronic AT1 receptor blockade in the normotensive rat.

Angiotensin And Osmoreceptor Inputs To The Area Postrema: Role In Long‐Term Control Of Fluid Homeostasis And Arterial Pressure

1. The role of the area postrema (AP) in the long‐term control of body fluid homeostasis and arterial pressure under conditions of increased dietary salt intake is reviewed. A model is proposed in which sympathetic nerve activity is suppressed when dietary salt is increased. It is hypothesized that the AP acts as an essential integrative site in the hind‐brain for this response.

Chronic effects of angiotensin II and at1 receptor antagonists in subfornical organ-lesioned rats.

1. Angiotensin (Ang) II is known to exert some of its effects centrally via circumventricular organs. These unique central nervous system areas lack the normal blood–brain barrier and, therefore, allow peptide hormones access to the brain. Of these, the subfornical organ (SFO) has been shown to be involved in many of the acute dipsogenic and pressor effects of AngII, but much less is known about the role of the SFO in the chronic effects of AngII. We hypothesized that the SFO is a central site involved in the chronic hypotensive effects of endogenous AT1 receptor blockade, as well as the chronic hypertensive effects of exogenously administered AngII.

Role of the Organum Vasculosum of the Lamina Terminalis for the Chronic Cardiovascular Effects Produced by Endogenous and Exogenous ANG II in Conscious Rats

Endogenous and exogenous circulating ANG II acts at one of the central circumventricular organs (CVOs), the subfornical organ (SFO), to modulate chronic blood pressure regulation. However, at the forebrain, another important CVO is the organum vasculosum of the lamina terminalis (OVLT). In the present study, we tested the hypothesis that the OVLT mediates the hypertension or the hypotension produced by chronic infusion of ANG II or losartan (AT1 antagonist), respectively. Six days after sham or OVLT electrolytic lesion, male Sprague-Dawley rats (280-320 g, n = 6 per group) were instrumented with intravenous catheters and radiotelemetric blood pressure transducers. Following another week of recovery, rats were given 3 days of saline control infusion (7 ml/day) and were then infused with ANG II (10 ng·kg(-1)·min(-1)) or losartan (10 mg·kg(-1)·day(-1)) for 10 days, followed by 3 recovery days. Twenty-four hour average measurements of mean arterial pressure (MAP) and heart rate (HR) were made during this protocol. Hydromineral balance (HB) responses were measured during the experimental protocol. By day 9 of ANG II treatment, MAP had increased 16 ± 4 mmHg in sham rats but only 4 ± 1 mmHg in OVLT lesioned rats without changes in HR or HB. However, the hypotension produced by 10 days of losartan infusion was not modified in OVLT lesioned rats. These results suggest that the OVLT might play an important role during elevation of plasma ANG II, facilitating increases of blood pressure but is not involved with baseline effects of endogenous ANG II.

Renal denervation attenuates long-term hypertensive effects of angiotensin II in the rat

1 It is well accepted that some of the long‐term effects of angiotensin (Ang) II are mediated via the central nervous system. Some of these actions that are mediated by the circumventricular organs and the baroreceptor reflex are thought to then alter sympathetic nervous system activity. In particular, there is some debate as to the role of renal nerves in the chronic effects of AngII. The aim of the present study was to assess the contribution of the renal nerves in a long‐term model of progressive AngII‐induced hypertension. 2 Male Sprague‐Dawley rats were subjected to either bilateral renal denervation (RDX; n = 7) or sham surgery (SHAM; n = 8). Rats were instrumented with radiotelemetric transducers and venous catheters for the measurement of blood pressure and AngII infusion, respectively. A 4.0% NaCl diet and distilled water were provided ad libitum. The first 3 days served as the control period (7 mL/day, 0.9% NaCl, i.v.). This was followed by an infusion of AngII for 16 days (10 ng/kg per min, i.v.) and a 3 day recovery period identical to control. 3 Baseline arterial pressure between RDX and SHAM rats did not differ. Following AngII treatment, the arterial pressure of SHAM rats increased more rapidly than that of RDX rats. By Day 10 of treatment, the mean arterial pressure was significantly different between groups, having increased to 166 ± 4 mmHg in SHAM rats and 135 ± 11 mmHg in RDX rats. This trend continued for the remainder of AngII treatment. 4 The present results indicate that the renal nerves are necessary for the full expression of AngII‐induced hypertension.

Effect of subfornical organ lesion on the development of mineralocorticoid-salt hypertension

Accumulating evidence suggests that structures within the lamina terminalis; the organum vasculosm of the lamina terminalis (OVLT), the median preoptic nucleus (MnPO) and/or the subfornical organ (SFO); are required for the development of DOCA-salt hypertension. Lesion of the anteroventral tissue lining the third ventricle (AV3V), which destroys cell bodies in the OVLT and MnPO, as well as efferent projections from the SFO to the OVLT and MnPO, abolishes DOCA-salt hypertension in the rat. However, the individual contribution of these structures to DOCA-salt hypertension is unknown. The present study was designed to determine whether an intact SFO is required for hypertension development in the DOCA-salt model. In uninephrectomized SFO lesioned (SFOx; n=6) and SHAM (n=8) Sprague-Dawley rats, 24-h mean arterial pressure (MAP) and heart rate (HR) were continuously recorded telemetrically 4 days before and 36 days after DOCA implantation (100 mg/rat; s.c.); 24-h sodium and water balances were measured throughout the protocol. No differences in control MAP, HR, sodium and water balances were observed between groups. Following DOCA implantation, the magnitude of the elevation of MAP was similar between groups (approximately 40 mm Hg) so that by Day 40, MAP was 148+/-5 mm Hg in SFOx and 145+/-4 mm Hg in SHAM rats. The magnitude of decrease in HR from control values was similar in both groups. Differences in sodium and water balances were not observed between groups. We conclude that the SFO alone does not play a significant role in the development of mineralocorticoid-salt hypertension.

The area postrema modulates hypothalamic Fos responses to intragastric hypertonic saline in conscious rats

We have recently reported that an acute intragastric hypertonic saline load increases Fos immunoreactivity in several central nuclei, including the supraoptic nucleus (SON), paraventricular nucleus (PVN), nucleus of the solitary tract (NTS), area postrema (AP), and lateral parabrachial nucleus (LPBN). We have also shown that these responses are mediated by stimulation of peripheral osmoreceptors with splanchnic and vagal afferent projections. However, it is unclear whether the primary projections of peripheral osmoreceptors terminate in the NTS or the AP, both of which project to the SON and PVN. This study tested the hypothesis that efferent projections from the AP were necessary for the Fos responses in the SON, PVN, and LPBN. We examined the effect of AP lesion on the response of central Fos immunoreactivity to intragastric hypertonic saline infusion in conscious rats. Compared with sham-lesioned rats (n = 5), Fos expression in AP-lesioned rats (n = 6) was similar in the SON following the intragastric sodium load. However, in contrast to the sham group, Fos expression was significantly reduced in the PVN of AP-lesioned rats. Fos levels observed in the NTS and LPBN were similar in both groups. These results suggest that the PVN response to intragastric hypertonic saline is dependent on efferent projections from the AP. In contrast, Fos responses to this stimulus in the NTS, SON, and LPBN are independent of the activity of the AP.